1 Field of Invention
The present invention relates to a semiconductor device fabrication. More particularly, the present invention relates to a method for fabricating a shallow trench isolation (STI) structure on a semiconductor substrate.
2. Description of Related Art
With the development of technologies in integrated circuits, device minimization and high integration become more and more important subjects in semiconductor device fabrication. Since the dimensions of devices are gradually reduced and the integration is gradually increased, isolation structures of devices are also minimized so that the technology of forming the isolation structures becomes more and more difficult. One method of forming the isolation structures is to form a field oxide layer by local oxidation (LOCOS). But the field oxide layer is restricted by its bird's beak shape, and so it is really difficult to minimize the dimensions of the field oxide layer. Accordingly, other device isolation structures have been gradually developed, of which shallow trench isolation (STI) structures are most widely used for device isolation. At present, the shallow trench isolation (STI) structures are widely used in semiconductor device fabrication at the sub-half micron level.
FIG. 1 is a schematic, cross-sectional view of a shallow trench isolation structure.
A method of forming a shallow trench isolation (STI) structure is to anisotropically etch a semiconductor substrate 100, using a silicon nitride layer (not shown) as a hard mask. A trench 110 is then formed in the substrate 100. A liner oxide layer 115 is formed on the trench 110 by thermal oxidation. A silicon oxide layer 120 is formed over the substrate 100 to fill the trench 110. The silicon nitride layer is removed. A device isolation structure is thus formed.
In STI process, rounding top corners 110a and bottom corners 110b of the trench 110 is the most important issue. If the top corners 110a and the bottom corners 110b of the trench 110 are not rounded, stress is generated at the top corners 110a and the bottom corners 110. Therefore, the top corners 110a and the bottom corners 110b of the trench 110 are rounded so that the stress and the electric field are minimized. Accordingly, the operating conditions for etching and thermal oxidation are controlled carefully. However, the results are still unreliable in the conventional method.